1. Field of the Invention
The present invention relates to an improvement in the process of producing maleic anhydride where a water scrubber is employed, by adding a stabilizing material to the scrubber water to reduce residue formation and loss of maleic anhydride during distillation separation of the water from the crude maleic composition.
2. Related Art
Maleic anhydride is currently produced by the oxidation of hydrocarbons such as benzene, n-butene, butadiene-1,3 or n-butane. Maleic anhydride is obtained by oxidizing the hydrocarbon feed at a high temperature and over a suitable catalyst to produce a gaseous effluent of maleic anhydride together with impurities. The gaseous effluent is cooled and scrubbed with water to produce a crude solution of maleic acid. The aqueous solution of maleic acid is then fed to a dehydration-distillation column in which the maleic acid is dehydrated by contacting with a volatile water insoluble entraining or azeotroping agent such as xylene which does not undergo chemical reaction in the system. The water and entraining agent are removed as overhead vapors and maleic anhydride is removed as bottoms. These processes for the oxidation of hydrocarbons to maleic anhydride, scrubbing of the maleic anhydride to produce an aqueous maleic acid solution and dehydration of the maleic acid solution to form maleic anhydride are known in the art and are described, for example, in Chemical and Engineering News 38, (28), 40, 1960; Encyclopedia of Polymer Science (1964); Kirk and Othmer Encyclopedia of Chemical Technology, 2nd. edition, Vol. 12, 828, Interscience (1967); U.S. Pat. No. 2,683,110 and U.S. Pat. No. 3,094,539. One of the disadvantages of these methods of dehydration is that a significant amount of maleic anhydride is lost as residue which is believed to be primarily fumaric acid and a conglomeration of higher molecular weight substances formed during the dehydration. The residue also produces fouling in the dehydration column.
The presence of sodium in the crude product is known to promote the rate of maleic anhydride decomposition and residue formation at elevated temperatures. Sodium may be introduced into the system with scrubber water makeup. U.S. Pat. No. 2,863,880 issued to Kohn found that phosphoric acid inhibited the alkali metal-induced decomposition of maleic anhydride, however, Kohn was dealing with what are considered relatively high amounts of alkali, i.e., 50 ppm or more. Moreover, Kohn stated that about six to eight equivalent acid groups per atomic equivalent of alkali metal ion was the maximum amount required to inhibit the system and that no advantage was observed for increased quantities.
That may be the case with large amounts of alkali metal, e.g., sodium, however, plants now generally operate with water of greater purity, with alkali content of 10 ppm or less. Surprisingly, it has been found for these low concentrations of alkali, much larger amounts of phosphoric acid are required to inhibit the alkali and the resultant residue formation.
Although, alkali ions are a major cause of maleic anhydride degradation during the dehydration, there are other factors, which cause residue. These other factors have not been identified. However, the use of phosphoric acid according to the present invention also reduces the residue formation of alkali free maleic anhydride due to these other factors.
It has also been found that a combination of phosphoric acid with certain other compositions provides a synergistic effect in the residue formation.